You dont want to turn the brightness up, you want that down. Its the contrast you want to turn up, and more so the blue colour, and this is what the gun will see.
I use a few different light guns on CRT projectors, and through a lot of testing, the biggest problems in order:
• not enough blue drive
• brightness too high ( washes out the screen )
• green too low
• red too bright
Using a CRT projector made it very easy to work out what the gun needs to see and what it does not.
• covering red lens did nothing
• covering green lens made the guns less likely to work at distance
• covering blue lens the guns did not function AT ALL no matter how close to the screen you got.
Using a direct view set you may need to access service menus to raise blue drive, but usually just raising contrast and lowering brightness is fine.
I see it so often people say "turn up the brightness"... Why??? The brightness controls black level or cutoff, which has very little to do with the gun, it sees a white or blue flash, which is a full 100ire, and contrast is the setting that affects that.